1. Field of the Invention
This invention generally relates to the operation of shock tubes for simulating free-field conditions at a test object and, more particularly, to an arrangement for counteracting, if not totally eliminating, the undesirable effects of a rarefaction wave on the test object.
2. Description of the Prior Art
A shock tube can be employed for determining the effects of an air blast on a test object located within the shock tube. A shock wave is generated with the driver section of the tube and, thereupon, the test object which is located within the test section of the tube, is subjected to the blast. The shock wave impinges on and travels past the test object in a downstream path along the shock tube toward and beyond the discharge or open end of the shock tube. As the shock wave expands out of the discharge end, a rarefaction wave is generated which travels away from the discharge end into and along the shock tube in a countercurrent upstream direction along the path to the test object. The rarefaction wave generates an underpressure or reduced pressure zone in the circumambient region of the test object.
When the effects of very large air blasts, for example, those caused by nuclear or non-nuclear supertonnage explosions, are to be investigated for a test object, one could utiize huge cylindrical shock tubes which are very long, e.g. on the order of 500 ft., and which are very wide, e.g. on the order of 8 ft. or more in diameter. Such huge shock tubes are, of course, very expensive to manufacture and even more expensive to install at a test site. Hence, it has also been proposed to employ shock tubes of reduced length to minimize such manufacturing and installation expenses. However, it has been found that the shorter the test section of the shock tube, the greater the magnitude and the effect of the rarefaction wave on the test object--a condition which unfortunately is undesirable since the presence of the rarefaction wave tends to destroy the free-field simulation.
More particularly, the presence of the rarefaction wave lowers the pressure in the circumambient region of the test object. In an ideal free-field simulation, the pressure condition or pressure curve at the test object rises abruptly to a maximum pressure value and thereupon decays smoothly at a predetermined rate to a zero pressure value. However, the rarefaction wave, which creates an underpressure at the test object, disturbs the aforementioned ideal pressure curve and, when the rarefaction wave arrives at the test object, the pressure at the test station can, in some cases, have a negative value. In addition, the rarefaction wave acts to increase the air flow upstream of the test object so that the flow or drag effect of the shock wave on the test object is greater than what was intended.
In an earlier attempt to eliminate the effect of the rarefaction wave, one method employed was to mount a flat, solid plate at, but slightly offset from, the discharge end of the shock tube. The solid plate generated a reflected wave which traveled into and along the shock tube toward the test object. However, there was little control over the magnitude of the reflected wave and/or the time when the reflected wave reached the test object. Hence, the rarefaction wave was not cancelled and, in fact, matters were made somewhat worse, inasmuch as now the pressure curve at the test object showed a sharp pressure jump when the reflected wave arrived at the test object.
Other approaches included the use of vented plates, perforated plates with absorbent material, solid plates with absorbent material, and single rows of bars. All of these approaches, however, are limited to the pressure range over which they can operate efficiently. Further, none of these approaches provide any adjustable control over the magnitude of the reflected wave and/or the time when the reflected wave reaches the test object.